The endogenous opioid peptides (i.e., enkephalins and dynorphins) in the hippocampal formation are important in learning and memory and are affected following seizures. Four studies are proposed to examine the cellular relationships of opioid-containing neurons in the rat (or where relevant the guinea pig) hippocampal formation. The proposed studies will employ electron microscopic dual labeling techniques for localizing (1) two antigens on single sections of rodent brain using combinations of immunoperoxidase, immunogold and immunoautoradiographic labels and (2) single antigens using immunocytochemical labels combined with histochemical or immunocytochemical demonstration of anterogradely transported tract-tracers or intracellularly-filled neurons. Study I will examine the normal cellular relationships between enkephalin- and dynorphin-containing neurons within three intrinsic systems: (a) the granule cells of the dentate gyrus including their mossy fiber axons; (b) perforant path terminals arising from the lateral entorhinal cortex; and (c) interneurons. Also, the hypothesis will be tested that opioid- containing mossy fibers change in either number and/or ultrastructure following kainic acid induced seizures. Study II will test the hypothesis that enkephalins excite pyramidal cells by synapsing on inhibitory interneurons containing gamma-aminobutyric acid (GABA). Additionally, the synaptic relations between dynorphin-containing and GABAergic neurons will be examined. Study III will examine the interactions between opioid-containing neurons and glutamatergic pathways, particularly whether (a) opioid-containing terminals synapse on granule and pyramidal cells, (b) opioids and glutamate are co- localized in mossy fibers; and (c) the synaptic relations between terminals arising from the entorhinal cortex and dynorphin-containing granule cells. Study IV will test the hypothesis that septal cholinergic and brainstem catecholaminergic terminals are synaptically related to opioid-containing neurons. The experiments will be carried out in adult male rodents, but could yield information that is directly applicable to devising improved therapeutic measures to manipulate transmitter systems disrupted by disorders, such as seizures, in humans. Moreover, such studies may provide insight into the opioid dependence and tolerance phenomena, which possibly reside in the opioid system itself or in the interactions of opioid neurons with neurons containing GABA, catecholamines or glutamate.